The present invention relates to methods of making preforms from which optical fibers can be drawn; these methods are especially useful for making high data rate optical fibers for telecommunication systems.
It has been known that solitons can be generated in optical fibers when the transmission power is in the nonlinear region. The optical soliton maintains its narrow temporal pulse as it propagates down the fiber because the dispersion is balanced with the nonlinear index. Mathematically this phenomenon is adequately described with the well known nonlinear Schroedinger equation. See, for example, the publication, C. Sien, "Concatenated Soliton Fibre Link", Electronics Letters, volume 12, pages 237-238 (1991). There are three important terms in the nonlinear Schroedinger equation. These terms relate to attenuation, the group velocity dispersion and the nonlinear index effects. The balancing of the group velocity dispersion with the nonlinear index term has received much attention to date and is well known. However, pulses propagating in real fibers undergo attenuation; this can cause soliton pulses to develop frequency chirping and subsequent broadening and to then become essentially linear.
As used herein the term "dispersion" means group velocity dispersion, which is the total of the material dispersion and the refractive index profile dispersion.
It has been proposed that a soliton can survive in a fiber with loss if the group velocity dispersion can be made to decrease approximately exponentially with distance (K. Tajima, "Compensation of Soliton Broadening in Nonlinear Optical Fibers with Loss", Optics Letters, volume 12(1), pp. 54-56, 1987). In this way, the group velocity dispersion is made to continuously change so that it matches the changing power level. That publication states that this can be accomplished by varying the core diameter through fiber tapering and that such a fiber can be manufactured by controlling the fiber draw speed. Such a fiber is illustrated in FIG. 1 wherein the diameter of fiber 5 exponentially decreases from the large diameter input end 6 to the small diameter output end 7. The diameter of the core of fiber 5 is proportional to the outside diameter of the fiber. In the theoretical example proposed by Tajima the effective core diameter of such a fiber changes exponentially from about 10 .mu.m to about 5 .mu.m over 100 km.
A dispersion decreasing fiber was actually made by varying the speed of the fiber draw to change the fiber outer diameter from 175 .mu.m to 115 .mu.m, whereby the measured dispersion decreased from 10 ps/nm-km to 1 ps/nm-km over a 1 km length (V. A. Bogatyrev et al., "A single-mode fiber with chromatic dispersion varying along the length", Journal of Lightwave Technology, volume 9(5), pages 561-566, 1991). Subsequently, that fiber was used to generate a continuous soliton pulse train at 70 Gb/s (S.V. Chernikov, "70 Gbit/s fibre based source of fundamental solitons at 1550 nm", Electronics Letters, volume 28(13), pages 1210-1211, 1992). Such fibers have potential application in ultrahigh bit rate telecommunication systems of the type schematically illustrated in the soliton communication system of FIG. 2. A pulse train is input to amplifier 11 and coupled to dispersion decreasing fiber DDF-15, the dispersion at the input end a being greater than that at output end b. After propagating a distance that is limited by the maximum dispersion change, the optical signal is again amplified at amplifier 12 and coupled to dispersion decreasing fiber DDF-16, which has a high dispersion end a adjacent amplifier 12 and a low dispersion end b adjacent amplifier 13.
It has also been suggested that dispersion-decreasing fiber may be useful in a soliton communication line to extend the distance between amplifiers 11 and 12, for example. While there clearly could be numerous applications of dispersion-decreasing fiber, a tapered fiber, in which the outside diameter as well as the core diameter changes to the extent proposed in the Tajima and Bogatyrev et al. publications, will result in problems with splicing and cabling, for example.